Two separate moose populations in Minnesota have fallen sharply in the last two decades, with one collapsing from 4,000 to under 100, and the other dropping by a quarter each year. The decline has alarmed scientists who have yet to identify a single cause for the dramatic drops, the New York Times notes.

Researchers say that a number of things could be raising moose mortality, including warmer winters, which increase the prevalence of parasites and heat stress. The introduction of wolves and increased hunting may also be playing a role in the die-offs. Moose are particularly vulnerable to ticks, which are more common during warmer winters.

A recent Canadian study found that moose populations in British Columbia were affected by the loss of many pine trees to pine bark beetles. Thinning forests left the moose with fewer areas to hide from hunters and predatory animals.

Fewer moose could impact local environments, including revenue from hunting permits and moose-watching tourism, which generates an estimated $115 million every year in New Hampshire.

Scientists are using radio collars to track moose in Minnesota. When a collared moose dies, the transmitter alerts researchers who arrive on scene to determine the cause of death.